Extended guard interval for outdoor WLAN

ABSTRACT

A wireless network interface device selects a guard interval from a set of guard intervals including a first guard interval, a second guard interval, and a third guard interval, where in the first guard interval has a length that is 50% of a length of the second guard interval, and wherein the length of the second guard interval is 50% of a length of the third guard interval. The wireless network interface device generates a preamble of a data unit to include: a legacy signal field, a repetition of the legacy field, and a non-legacy field that includes a field that indicates the selected guard interval. The wireless network interface device generates a data portion of the data unit, including generating orthogonal frequency division multiplexing (OFDM) symbols of the data portion using the selected guard interval.

CROSS-REFERENCES TO RELATED APPLICATIONS

This disclosure is a continuation of U.S. application Ser. No.14/483,106, now U.S. Pat. No. 9,294,323, entitled “Extended GuardInterval for Outdoor WLAN,” filed Sep. 10, 2014, which claims thebenefit of U.S. Provisional Patent Application No. 61/875,968, entitled“Longer GI for Outdoor,” filed on Sep. 10, 2013, and U.S. ProvisionalPatent Application No. 61/911,232, entitled “Longer GI for Outdoor,”filed on Dec. 3, 2013, the disclosures of all of which are incorporatedherein by reference in their entireties.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to communication networks and,more particularly, to wireless local area networks that utilizeorthogonal frequency division multiplexing (OFDM).

BACKGROUND

When operating in an infrastructure mode, wireless local area networks(WLANs) typically include an access point (AP) and one or more clientstations. WLANs have evolved rapidly over the past decade. Developmentof WLAN standards such as the Institute for Electrical and ElectronicsEngineers (IEEE) 802.11a, 802.11b, 802.11g, and 802.11n Standards hasimproved single-user peak data throughput. For example, the IEEE 802.11bStandard specifies a single-user peak throughput of 11 megabits persecond (Mbps), the IEEE 802.11a and 802.11g Standards specify asingle-user peak throughput of 54 Mbps, the IEEE 802.11n Standardspecifies a single-user peak throughput of 600 Mbps, and the IEEE802.11ac Standard specifies a single-user peak throughput in thegigabits per second (Gbps) range. Future standards promise to provideeven greater throughputs, such as throughputs in the tens of Gbps range.

SUMMARY

In an embodiment, a method for generating a data unit for transmissionvia a wireless communication channel includes selecting, at a wirelessnetwork interface device, a guard interval from a set of guard intervalsincluding a first guard interval, a second guard interval, and a thirdguard interval, where in the first guard interval has a length that is50% of a length of the second guard interval, and wherein the length ofthe second guard interval is 50% of a length of the third guardinterval. The method also includes generating, at the wireless networkinterface device, a preamble of a data unit to include: a legacy signalfield, a repetition of the legacy field, and a non-legacy field thatincludes a field that indicates the selected guard interval. The methodadditionally includes generating, at the wireless network interfacedevice, a data portion of the data unit, including generating orthogonalfrequency division multiplexing (OFDM) symbols of the data portion usingthe selected guard interval.

In another embodiment, an apparatus comprises a wireless networkinterface device having one or more integrated circuits. The one or moreintegrated circuits are configured to select a guard interval from a setof guard intervals including a first guard interval, a second guardinterval, and a third guard interval, where in the first guard intervalhas a length that is 50% of a length of the second guard interval, andwherein the length of the second guard interval is 50% of a length ofthe third guard interval. The one or more integrated circuits are alsoconfigured to generate a preamble of a data unit to include: a legacysignal field, a repetition of the legacy field, and a non-legacy fieldthat includes a field that indicates the selected guard interval. Theone or more integrated circuits are further configured to generate adata portion of the data unit, including generating orthogonal frequencydivision multiplexing (OFDM) symbols of the data portion using theselected guard interval.

In yet another embodiment, a tangible, non-transitory computer readablemedium, or media, stores machine readable instructions that, whenexecuted by one or more processors, cause the one or more processors toselect a guard interval from a set of guard intervals including a firstguard interval, a second guard interval, and a third guard interval,where in the first guard interval has a length that is 50% of a lengthof the second guard interval, and wherein the length of the second guardinterval is 50% of a length of the third guard interval. Theinstructions also cause the one or more processors to cause a wirelessnetwork interface device to generate a preamble of a data unit toinclude: a legacy signal field, a repetition of the legacy field, and anon-legacy signal field that includes a field that indicates theselected guard interval. The instructions further cause the one or moreprocessors to: cause the wireless network interface device to generate adata portion of the data unit, including generating orthogonal frequencydivision multiplexing (OFDM) symbols of the data portion using theselected guard interval; and cause the wireless network interface deviceto transmit the data unit via a wireless communication channel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example wireless local area network(WLAN) 10, according to an embodiment;

FIGS. 2A and 2B are diagrams of a prior art data unit format;

FIG. 3 is a diagram of another prior art data unit format;

FIG. 4 is a diagram of another prior art data unit format;

FIG. 5 is a diagram of another prior art data unit format;

FIG. 6A are diagrams of modulation used to modulate symbols in a priorart data unit;

FIG. 6B are diagrams of modulation used to modulate symbols in anexample data unit, according to an embodiment;

FIG. 7A is a diagram of an orthogonal frequency division multiplexing(OFDM) data unit, according to an embodiment.

FIG. 7B are diagrams of modulation used to modulate symbols in the dataunit depicted in FIG. 7A, according to an embodiment;

FIG. 8 is a block diagram of an OFDM symbol, according to an embodiment.

FIG. 9A is a diagram illustrating an example data unit in which thenormal guard interval is used for a preamble of the data unit, accordingto an embodiment.

FIG. 9B is a diagram illustrating an example data unit in which thenormal guard interval is used for only a portion a preamble of the dataunit, according to an embodiment.

FIG. 10A is a diagram illustrating an example data unit in which OFDMtone spacing is used to effectively increase guard interval duration,according to an embodiment.

FIG. 10B is a diagram illustrating an example data unit in which OFDMtone spacing is used to effectively increase guard interval duration,according to another embodiment.

FIG. 11A is a diagram illustrating a regular guard interval mode dataunit, according to an embodiment.

FIG. 11B is a diagram illustrating an extension guard interval mode dataunit, according to an embodiment.

FIGS. 12A-12B are diagrams illustrating two possible formats of a longtraining field, according to two example embodiments.

FIG. 13A is a diagram illustrating a non-legacy signal field of theregular guard interval mode data unit of FIG. 11A, according to anembodiment.

FIG. 13B is a diagram illustrating a non-legacy signal field of theextension guard interval mode data unit of FIG. 11B, according to anembodiment.

FIG. 14A is a block diagram illustrating an extension guard intervalmode data unit, according to an embodiment.

FIG. 14B is a diagram illustrating a legacy signal field of theextension guard interval data unit of FIG. 14A, according to oneembodiment.

FIG. 14C is a diagram illustrating a Fast Fourier Transform (FFT) windowfor the legacy signal field of FIG. 14B at the legacy receiving device,according to an embodiment.

FIG. 15 is a block diagram illustrating format of a non-legacy signalfield, according to an embodiment.

FIG. 16 is a flow diagram of a method for generating a data unit,according to an embodiment.

FIG. 17 is a flow diagram of a method for generating a data unit,according to another embodiment.

DETAILED DESCRIPTION

In embodiments described below, a wireless network device such as anaccess point (AP) of a wireless local area network (WLAN) transmits datastreams to one or more client stations. The AP is configured to operatewith client stations according to at least a first communicationprotocol. The first communication protocol is sometimes referred hereinas “high efficiency WiFi” or “HEW” communication protocol. In someembodiments, different client stations in the vicinity of the AP areconfigured to operate according to one or more other communicationprotocols which define operation in the same frequency band as the HEWcommunication protocol but with generally lower data throughputs. Thelower data throughput communication protocols (e.g., IEEE 802.11a, IEEE802.11n, and/or IEEE 802.11ac) are collectively referred herein as“legacy” communication protocols. In at least some embodiments, thelegacy communication protocols are generally deployed in indoorcommunication channels, and the HEW communication protocol is at leastsometimes deployed for outdoor communications.

According to an embodiment, symbols transmitted by the AP include guardintervals to prevent or minimize intersymbol interference at thereceiver caused by multipath propagation in the communication channel.The length of the guard interval needed to mitigate interferencegenerally depends on the delay spread of the particular channel beingutilized. For example, an outdoor communication channel is typicallycharacterized by a greater channel delay spread compared to an indoorcommunication channel, in at least some embodiments and/or scenarios. Inan embodiment, the HEW communication protocol defines a regular guardinterval mode and an extension guard interval mode. The regular guardinterval mode is generally used with communication channelscharacterized by shorter channel delay spreads (e.g., indoorcommunication channels), while the extension guard interval mode isgenerally used with communication channels characterized by relativelylonger channel delay spreads (e.g., outdoor communication channels), inan embodiment. In an embodiment, a normal guard interval (NGI) or ashort guard interval (SGI) is used in the regular guard interval mode,and a long guard interval (LGI) is used in the extension guard intervalmode.

In an embodiment, a data unit transmitted by the AP includes a preambleand a data portion, wherein the preamble is used, at least in part, tosignal, to a receiving device, various parameters used for transmissionof the data portion. In various embodiments, the preamble of a data unitis used to signal, to a receiving device, the particular guard intervalbeing utilized in at least the data portion of the data unit. In someembodiments, a same preamble format is used in the regular guardinterval mode an in the extension guard interval mode. In one suchembodiment, the preamble includes an indication set to indicate whetherthe NGI, the SGI or the LGI is used for at least the data portion of thedata unit. In some embodiments, the indicated NGI, SGI or LGI is usedfor at least a portion of the preamble of the data unit, in addition tothe data portion of the data unit. In an embodiment, the receivingdevice determines the particular guard interval being utilized based onthe indication in the preamble of the data unit, and then decodes theappropriate remaining portion of the data unit (e.g., the data portion,or a portion of the preamble and the data portion) using the particularguard interval.

In another embodiment, a preamble used in the extension guard intervalmode is formatted differently from a preamble used in the regular guardinterval mode. For example, the preamble used in the extension guardinterval mode is formatted such that the receiving device canautomatically (e.g., prior to decoding) detect that the data unitcorresponds to the extended guard interval mode. In an embodiment, whenthe receiving device detects that the data unit corresponds to theextended guard interval mode, the receiving device decodes the dataportion of the data unit, and in at least some embodiments, at least aportion of the preamble as well as the data portion of the data unit,using the LGI. On the other hand, when the receiving device detects thatthe data unit does not correspond to the extended guard interval mode,the receiving device assumes that the data unit corresponds to theregular guard interval mode, in an embodiment. The receiving device thendetermines, for example based on an indication in the preamble, whetherthe NGI or the SGI is used in the data unit, and decodes at least thedata portion of the data unit using the NGI or the SGI according to thedetermination, in an embodiment.

Additionally, in at least some embodiment, a preamble of a data unit inthe regular guard interval mode and/or in the extension guard intervalmode is formatted such that a client station that operates according toa legacy protocol, and not the HEW communication protocol, is able todetermine certain information regarding the data unit, such as aduration of the data unit, and/or that the data unit does not conform tothe legacy protocol. Additionally, a preamble of the data unit isformatted such that a client station that operates according to the HEWprotocol is able to determine the data unit conforms to the HEWcommunication protocol, in an embodiment. Similarly, a client stationconfigured to operate according to the HEW communication protocol alsotransmits data units such as described above, in an embodiment.

In at least some embodiments, data units formatted such as describedabove are useful, for example, with an AP that is configured to operatewith client stations according to a plurality of different communicationprotocols and/or with WLANs in which a plurality of client stationsoperate according to a plurality of different communication protocols.Continuing with the example above, a communication device configured tooperate according to both the HEW communication protocol and a legacycommunication protocol is able to determine that the data unit isformatted according to the HEW communication protocol and not the legacycommunication protocol. Similarly, a communication device configured tooperate according to a legacy communication protocol but not the HEWcommunication protocol is able to determine that the data unit is notformatted according to the legacy communication protocol and/ordetermine a duration of the data unit.

FIG. 1 is a block diagram of an example wireless local area network(WLAN) 10, according to an embodiment. An AP 14 includes a hostprocessor 15 coupled to a network interface 16. The network interface 16includes a medium access control (MAC) processing unit 18 and a physicallayer (PHY) processing unit 20. The PHY processing unit 20 includes aplurality of transceivers 21, and the transceivers 21 are coupled to aplurality of antennas 24. Although three transceivers 21 and threeantennas 24 are illustrated in FIG. 1, the AP 14 includes other suitablenumbers (e.g., 1, 2, 4, 5, etc.) of transceivers 21 and antennas 24 inother embodiments. In one embodiment, the MAC processing unit 18 and thePHY processing unit 20 are configured to operate according to a firstcommunication protocol (e.g., HEW communication protocol). In anotherembodiment, the MAC processing unit 18 and the PHY processing unit 20are also configured to operate according to a second communicationprotocol (e.g., IEEE 802.11ac Standard). In yet another embodiment, theMAC processing unit 18 and the PHY processing unit 20 are additionallyconfigured to operate according to the second communication protocol, athird communication protocol and/or a fourth communication protocol(e.g., the IEEE 802.11a Standard and/or the IEEE 802.11n Standard).

The WLAN 10 includes a plurality of client stations 25. Although fourclient stations 25 are illustrated in FIG. 1, the WLAN 10 includes othersuitable numbers (e.g., 1, 2, 3, 5, 6, etc.) of client stations 25 invarious scenarios and embodiments. At least one of the client stations25 (e.g., client station 25-1) is configured to operate at leastaccording to the first communication protocol. In some embodiments, atleast one of the client stations 25 is not configured to operateaccording to the first communication protocol but is configured tooperate according to at least one of the second communication protocol,the third communication protocol and/or the fourth communicationprotocol (referred to herein as a “legacy client station”).

The client station 25-1 includes a host processor 26 coupled to anetwork interface 27. The network interface 27 includes a MAC processingunit 28 and a PHY processing unit 29. The PHY processing unit 29includes a plurality of transceivers 30, and the transceivers 30 arecoupled to a plurality of antennas 34. Although three transceivers 30and three antennas 34 are illustrated in FIG. 1, the client station 25-1includes other suitable numbers (e.g., 1, 2, 4, 5, etc.) of transceivers30 and antennas 34 in other embodiments.

According to an embodiment, the client station 25-4 is a legacy clientstation, i.e., the client station 25-4 is not enabled to receive andfully decode a data unit that is transmitted by the AP 14 or anotherclient station 25 according to the first communication protocol.Similarly, according to an embodiment, the legacy client station 25-4 isnot enabled to transmit data units according to the first communicationprotocol. On the other hand, the legacy client station 25-4 is enabledto receive and fully decode and transmit data units according to thesecond communication protocol, the third communication protocol and/orthe fourth communication protocol.

In an embodiment, one or both of the client stations 25-2 and 25-3, hasa structure the same as or similar to the client station 25-1. In anembodiment, the client station 25-4 has a structure similar to theclient station 25-1. In these embodiments, the client stations 25structured the same as or similar to the client station 25-1 have thesame or a different number of transceivers and antennas. For example,the client station 25-2 has only two transceivers and two antennas,according to an embodiment.

In various embodiments, the PHY processing unit 20 of the AP 14 isconfigured to generate data units conforming to the first communicationprotocol and having formats described herein. The transceiver(s) 21is/are configured to transmit the generated data units via theantenna(s) 24. Similarly, the transceiver(s) 24 is/are configured toreceive the data units via the antenna(s) 24. The PHY processing unit 20of the AP 14 is configured to process received data units conforming tothe first communication protocol and having formats describedhereinafter and to determine that such data units conform to the firstcommunication protocol, according to various embodiments.

In various embodiments, the PHY processing unit 29 of the client device25-1 is configured to generate data units conforming to the firstcommunication protocol and having formats described herein. Thetransceiver(s) 30 is/are configured to transmit the generated data unitsvia the antenna(s) 34. Similarly, the transceiver(s) 30 is/areconfigured to receive data units via the antenna(s) 34. The PHYprocessing unit 29 of the client device 25-1 is configured to processreceived data units conforming to the first communication protocol andhaving formats described hereinafter and to determine that such dataunits conform to the first communication protocol, according to variousembodiments.

FIG. 2A is a diagram of a prior art OFDM data unit 200 that the AP 14 isconfigured to transmit to the client station 25-4 via orthogonalfrequency division multiplexing (OFDM) modulation, according to anembodiment. In an embodiment, the client station 25-4 is also configuredto transmit the data unit 200 to the AP 14. The data unit 200 conformsto the IEEE 802.11a Standard and occupies a 20 Megahertz (MHz) band. Thedata unit 200 includes a preamble having a legacy short training field(L-STF) 202, generally used for packet detection, initialsynchronization, and automatic gain control, etc., and a legacy longtraining field (L-LTF) 204, generally used for channel estimation andfine synchronization. The data unit 200 also includes a legacy signalfield (L-SIG) 206, used to carry certain physical layer (PHY) parametersof with the data unit 200, such as modulation type and coding rate usedto transmit the data unit, for example. The data unit 200 also includesa data portion 208. FIG. 2B is a diagram of example data portion 208(not low density parity check encoded), which includes a service field,a scrambled physical layer service data unit (PSDU), tail bits, andpadding bits, if needed. The data unit 200 is designed for transmissionover one spatial or space-time stream in a single input single output(SISO) channel configuration.

FIG. 3 is a diagram of a prior art OFDM data unit 300 that the AP 14 isconfigured to transmit to the client station 25-4 via orthogonalfrequency domain multiplexing (OFDM) modulation, according to anembodiment. In an embodiment, the client station 25-4 is also configuredto transmit the data unit 300 to the AP 14. The data unit 300 conformsto the IEEE 802.11n Standard, occupies a 20 MHz band, and is designedfor mixed mode situations, i.e., when the WLAN includes one or moreclient stations that conform to the IEEE 802.11a Standard but not theIEEE 802.11n Standard. The data unit 300 includes a preamble having anL-STF 302, an L-LTF 304, an L-SIG 306, a high throughput signal field(HT-SIG) 308, a high throughput short training field (HT-STF) 310, and Mdata high throughput long training fields (HT-LTFs) 312, where M is aninteger generally determined by the number of spatial streams used totransmit the data unit 300 in a multiple input multiple output (MIMO)channel configuration. In particular, according to the IEEE 802.11nStandard, the data unit 300 includes two HT-LTFs 312 if the data unit300 is transmitted using two spatial streams, and four HT-LTFs 312 isthe data unit 300 is transmitted using three or four spatial streams. Anindication of the particular number of spatial streams being utilized isincluded in the HT-SIG field 308. The data unit 300 also includes a dataportion 314.

FIG. 4 is a diagram of a prior art OFDM data unit 400 that the AP 14 isconfigured to transmit to the client station 25-4 via orthogonalfrequency domain multiplexing (OFDM) modulation, according to anembodiment. In an embodiment, the client station 25-4 is also configuredto transmit the data unit 400 to the AP 14. The data unit 400 conformsto the IEEE 802.11n Standard, occupies a 20 MHz band, and is designedfor “Greenfield” situations, i.e., when the WLAN does not include anyclient stations that conform to the IEEE 802.11a Standard but not theIEEE 802.11n Standard. The data unit 400 includes a preamble having ahigh throughput Greenfield short training field (HT-GF-STF) 402, a firsthigh throughput long training field (HT-LTF1) 404, a HT-SIG 406, and Mdata HT-LTFs 408, where M is an integer which generally corresponds to anumber of spatial streams used to transmit the data unit 400 in amultiple input multiple output (MIMO) channel configuration. The dataunit 400 also includes a data portion 410.

FIG. 5 is a diagram of a prior art OFDM data unit 500 that the clientstation AP 14 is configured to transmit to the client station 25-4 viaorthogonal frequency domain multiplexing (OFDM) modulation, according toan embodiment. In an embodiment, the client station 25-4 is alsoconfigured to transmit the data unit 500 to the AP 14. The data unit 500conforms to the IEEE 802.11ac Standard and is designed for “Mixed field”situations. The data unit 500 occupies a 20 MHz bandwidth. In otherembodiments or scenarios, a data unit similar to the data unit 500occupies a different bandwidth, such as a 40 MHz, an 80 MHz, or a 160MHz bandwidth. The data unit 500 includes a preamble having an L-STF502, an L-LTF 504, an L-SIG 506, two first very high throughput signalfields (VHT-SIGAs) 508 including a first very high throughput signalfield (VHT-SIGA1) 508-1 and a second very high throughput signal field(VHT-SIGA2) 508-2, a very high throughput short training field (VHT-STF)510, M very high throughput long training fields (VHT-LTFs) 512, where Mis an integer, and a second very high throughput signal field(VHT-SIG-B) 514. The data unit 500 also includes a data portion 516.

FIG. 6A is a set of diagrams illustrating modulation of the L-SIG,HT-SIG1, and HT-SIG2 fields of the data unit 300 of FIG. 3, as definedby the IEEE 802.11n Standard. The L-SIG field is modulated according tobinary phase shift keying (BPSK), whereas the HT-SIG1 and HT-SIG2 fieldsare modulated according to BPSK, but on the quadrature axis (Q-BPSK). Inother words, the modulation of the HT-SIG1 and HT-SIG2 fields is rotatedby 90 degrees as compared to the modulation of the L-SIG field.

FIG. 6B is a set of diagrams illustrating modulation of the L-SIG,VHT-SIGA1, and VHT-SIGA2 fields of the data unit 500 of FIG. 5, asdefined by the IEEE 802.11ac Standard. Unlike the HT-SIG1 field in FIG.6A, the VHT-SIGA1 field is modulated according to BPSK, same as themodulation of the L-SIG field. On the other hand, the VHT-SIGA2 field isrotated by 90 degrees as compared to the modulation of the L-SIG field.

FIG. 7A is a diagram of an OFDM data unit 700 that the client station AP14 is configured to transmit to the client station 25-1 via orthogonalfrequency domain multiplexing (OFDM) modulation, according to anembodiment. In an embodiment, the client station 25-1 is also configuredto transmit the data unit 700 to the AP 14. The data unit 700 conformsto the first communication protocol and occupies a 20 MHz bandwidth.Data units similar to the data unit 700 occupy other suitable bandwidthsuch as 40 MHz, 80 MHz, 160 MHz, 320 MHz, 640 MHz, for example, or othersuitable bandwidths, in other embodiments. The data unit 700 is suitablefor “mixed mode” situations, i.e. when the WLAN 10 includes a clientstation (e.g., the legacy client station 24-4) that conforms to a legacycommunication protocol, but not the first communication protocol. Thedata unit 700 is utilized in other situations as well, in someembodiments.

The data unit 700 includes a preamble 701 having an L-STF 702, an L-LTF704, an L-SIG 706, two first HEW signal fields (HEW-SIGAs) 708 includinga first HEW signal field (HEW-SIGA1) 708-1 and a second HEW signal field(HEW-SIGA2) 708-2, an HEW short training field (HEW-STF) 710, M HEW longtraining fields (HEW-LTFs) 712, where M is an integer, and a third HEWsignal field (HEW-SIGB) 714. Each of the L-STF 702, the L-LTF 704, theL-SIG 706, the HEW-SIGAs 708, the HEW-STF 710, the M HEW-LTFs 712, andthe HEW-SIGB 714 comprises an integer number of one or more OFDMsymbols. For example, in an embodiment, the HEW-SIGAs 708 comprise twoOFDM symbols, where the HEW-SIGA1 708-1 field comprises the first OFDMsymbol and the HEW-SIGA2 comprises the second OFDM symbol. In at leastsome examples, the HEW-SIGAs 708 are collectively referred to as asingle HEW signal field (HEW-SIGA) 708. In some embodiments, the dataunit 700 also includes a data portion 716. In other embodiments, thedata unit 700 omits the data portion 716.

In the embodiment of FIG. 7A, the data unit 700 includes one of each ofthe L-STF 702, the L-LTF 704, the L-SIG 706, the HEW-SIGA1 s 708. Inother embodiments in which an OFDM data unit similar to the data unit700 occupies a cumulative bandwidth other than 20 MHz, each of the L-STF702, the L-LTF 704, the L-SIG 706, the HEW-SIGA1s 708 is repeated over acorresponding number of 20 MHz sub-bands of the whole bandwidth of thedata unit, in an embodiment. For example, in an embodiment, the OFDMdata unit occupies an 80 MHz bandwidth and, accordingly, includes fourof each of the L-STF 702, the L-LTF 704, the L-SIG 706, the HEW-SIGA1s708, in an embodiment. In some embodiments, the modulation of different20 MHz sub-bands signals is rotated by different angles. For example, inone embodiment, a first subband is rotated 0-degrees, a second subbandis rotated 90-degrees, a third sub-band is rotated 180-degrees, and afourth sub-band is rotated 270-degrees. In other embodiments, differentsuitable rotations are utilized. The different phases of the 20 MHzsub-band signals result in reduced peak to average power ratio (PAPR) ofOFDM symbols in the data unit 700, in at least some embodiments. In anembodiment, if the data unit that conforms to the first communicationprotocol is an OFDM data unit that occupies a cumulative bandwidth suchas 20 MHz, 40 MHz, 80 MHz, 160 MHz, 320 MHz, 640 MHz, etc., the HEW-STF,the HEW-LTFs, the HEW-SIGB and the HEW data portion occupy thecorresponding whole bandwidth of the data unit.

FIG. 7B is a set of diagrams illustrating modulation of the L-SIG 706,HEW-SIGA1 708-1, and HEW-SIGA2 708-2 of the data unit 700 of FIG. 7A,according to an embodiment. In this embodiment, the L-SIG 706, HEW-SIGA1708-1, and HEW-SIGA2 708-2 fields have the same modulation as themodulation of the corresponding field as defined in the IEEE 802.11acStandard and depicted in FIG. 6B. Accordingly, the HEW-SIGA1 field ismodulated the same as the L-SIG field. On the other hand, the HEW-SIGA2field is rotated by 90 degrees as compared to the modulation of theL-SIG field.

In an embodiment, because the modulations of the L-SIG 706, HEW-SIGA1708-1, and HEW-SIGA2 708-2 fields of the data unit 700 correspond to themodulations of the corresponding fields in a data unit that conforms tothe IEEE 802.11ac Standard (e.g., the data unit 500 of FIG. 5), legacyclient stations configured to operate according to the IEEE 802.11aStandard and/or the IEEE 802.11n Standard will assume, in at least somecircumstances, that the data unit 700 conforms to the IEEE 802.11acStandard and will process the data unit 700 accordingly. For example, aclient station the conforms to the IEEE 802.11a Standard will recognizethe legacy IEEE 802.11a Standard portion of the preamble of the dataunit 700 and will set the data unit duration according to a durationindicated in the L-SIG 706. For example, the legacy client station willcalculate a duration based on a rate and a length (e.g., in number ofbytes) indicated in the L-SIG field 706, according to an embodiment. Inan embodiment, the rate and the length in the L-SIG field 706 are setsuch that a client station configured to operate according to a legacycommunication protocol will calculate, based the rate and the length, apacket duration (T) that corresponds to, or at least approximates, theactual duration of the data unit 700. For example, the rate is set toindicate a lowest rate defined by the IEEE 802.11a Standard (i.e., 6Mbps), and the length is set to a value computed such that packetduration computed using the lowest rate at least approximates the actualduration of the data unit 700, in one embodiment.

In an embodiment, a legacy client station that conforms to the IEEE802.11a Standard, when receiving the data unit 700, will compute apacket duration for the data unit 700, e.g., using a rate and a lengthfields of L-SIG field 706, and will wait until the end of the computedpacket duration before performing clear channel assessment (CCA), in anembodiment. Thus, in this embodiment, communication medium is protectedagainst access by the legacy client station at least for the duration ofthe data unit 700. In an embodiment, the legacy client station willcontinue decoding the data unit 700, but will fail an error check (e.g.,using a frame check sequence (FCS)) at the end of the data unit 700.

Similarly, a legacy client station configured to operate according tothe IEEE 802.11n Standard, when receiving the data unit 700, willcompute a packet duration (T) of the data unit 700 based on the rate andthe length indicated in the L-SIG 706 of the data unit 700, in anembodiment. The legacy client station will detect the modulation of thefirst HEW signal field (HEW-SIGA1) 708-1 (BPSK) and will assume that thedata unit 700 is a legacy data unit that conforms to the IEEE 802.11aStandard. In an embodiment, the legacy client station will continuedecoding the data unit 700, but will fail an error check (e.g., using aframe check sequence (FCS)) at the end of the data unit. In any event,according to the IEEE 802.11n Standard, the legacy client station willwait until the end of a computed packet duration (T) before performingclear channel assessment (CCA), in an embodiment. Thus, communicationmedium will be protected from access by the legacy client station forthe duration of the data unit 700, in an embedment.

A legacy client station configured to operate according to the IEEE802.11ac Standard but not the first communication protocol, whenreceiving the data unit 700, will compute a packet duration (T) of thedata unit 700 based on the rate and the length indicated in the L-SIG706 of the data unit 700, in an embodiment. However, the legacy clientstation will not be able to detect, based on the modulation of the dataunit 700, that the data unit 700 does not conform to the IEEE 802.11acStandard, in an embodiment. In some embodiments, one or more HEW signalfields (e.g., the HEW-SIGA1 and/or the HEW-SIGA2) of the data unit 700is/are formatted to intentionally cause the legacy client station todetect an error when decoding the data unit 700, and to therefore stopdecoding (or “drop”) the data unit 700. For example, HEW-SIGA 708 of thedata unit 700 is formatted to intentionally cause an error when the SIGAfield is decoded by a legacy device according to the IEEE 802.11acStandard, in an embodiment. Further, according to the IEEE 802.11acStandard, when an error is detected in decoding the VHT-SIGA field, theclient station will drop the data unit 700 and will wait until the endof a computed packet duration (T), calculated, for example, based on arate and a length indicated in the L-SIG 706 of the data unit 700,before performing clear channel assessment (CCA), in an embodiment.Thus, communication medium will be protected from access by the legacyclient station for the duration of the data unit 700, in an embedment.

FIG. 8 is a diagram of an OFDM symbol 800, according to an embodiment.The data unit 700 of FIG. 7 includes OFDM symbols such as the OFDMsymbols 800, in an embodiment. The OFDM symbol 800 includes a guardinterval portion 802 and an information portion 804. In an embodiment,the guard interval comprises a cyclic prefix repeating an end portion ofthe OFDM symbol. In an embodiment, the guard interval portion 802 isused to ensure orthogonality of OFDM tones at a receiving device (e.g.,the client station 25-1) and to minimize or eliminate inter-symbolinterference due to multi-path propagation in the communication channelvia which the OFDM symbol 800 is transmitted from a transmitting device(e.g., the AP 14) to the receiving device. In an embodiment, the lengthof the guard interval portion 802 is selected based on expected worstcase channel delay spread in the communication channel between thetransmitting device and the receiving device. For example, a longerguard interval is selected for outdoor communication channels typicallycharacterized by longer channel delay spreads as compared to a shorterguard interval selected for indoor communication channels typicallycharacterized by shorter channel delay spreads, in an embodiment.

According to an embodiment, the guard interval portion 802 correspondsto a short guard interval, a normal guard interval, or a long guardinterval, depending on mode of transmission being utilized. In anembodiment, the short guard interval or the normal guard interval isused for indoor communication channels or communication channels withrelatively short channel delay spreads, and the long guard interval isused for outdoor communication channels or communication channels withrelatively long delay spreads. In an embodiment, the normal guardinterval or the short guard interval is used for some or all OFDMsymbols of an HEW data unit (e.g., the HEW data unit 700) when the HEWdata unit is transmitted in regular guard interval mode, and the longguard interval is used for at least some OFDM symbols of the HEW dataunit when the HEW data unit is transmitted in extension guard intervalmode.

In an embodiment, the short guard interval (SGI) has a length of 0.4 μs,the normal guard interval is 0.8 μs and the long guard interval (LGI)has a length of 1.2 μs or 1.8 μs. In an embodiment, the informationportion 804 has a length of 3.2 μs. In other embodiments, other suitablelengths for the SGI, the NGI, the LGI, and/or the information portion804 are utilized. In some embodiments, the SGI has a length that is 50%of the length of the NGI, and the NGI has a length that is 50% of thelength of the LGI. In other embodiments, the SGI has a length that is75% or less of the length of the NGI, and the NGI has a length that is75% or less of the length of the LGI. In other embodiments, the SGI hasa length that is 50% or less of the length of the NGI, and the NGI has alength that is 50% or less of the LGI.

In some embodiments, the extension guard interval mode uses the normalguard interval duration of the regular guard interval mode, but uses adifferent OFDM modulation that effectively extends the guard intervalduration in the extension guard interval mode. For example, in anembodiment, OFDM modulation with reduced tone spacing is used in theextension guard interval mode. For example, whereas the regular guardinterval mode for a 20 MHz bandwidth OFDM data unit uses a 64-pointdiscrete Fourier transform (DFT), resulting in 64 OFDM tones, theextension guard interval mode uses a 128-point DFT for a 20 MHz OFDMdata unit, resulting in 128 OFDM tones in the same bandwidth. In thiscase, tone spacing in the extension guard interval mode OFDM symbols isreduced by a factor of two (½) compared to regular guard interval modeOFDM symbols. As another example, whereas the regular guard intervalmode for a 20 MHz bandwidth OFDM data unit uses a 64-point discreteFourier transform (DFT) resulting in 64 OFDM tones, the extension guardinterval mode uses a 256-point DFT for a 20 MHz OFDM data unit resultingin 256 OFDM tones in the same bandwidth. In this case, tone spacing inthe extension guard interval mode OFDM symbols is reduced by a factor offour (¼) compared to the regular guard interval mode OFDM symbols. Insuch embodiments, long GI duration of, for example, 1.6 μs is used.However, the duration of the information portion of the extension guardinterval mode OFDM symbol is increased (e.g., from 3.2 μs to 6.4 μs),and the percentage of the GI portion duration to the total OFDM symbolsduration remains the same, in an embodiment. Thus, in this case, loss ofefficiency due to a longer GI symbol is avoided, in at least someembodiments. In various embodiments, the term “long guard interval” asused herein encompasses an increased duration of a guard interval aswell as a decreased OFDM tone spacing that effectively increasesduration of the guard interval.

FIG. 9A is a diagram illustrating an example data unit 900 in which thenormal guard interval is used for a preamble of the data unit, accordingto an embodiment. The data unit 900 is generally the same as the dataunit 700 of FIG. 7A and includes like-numbered elements with the dataunit 700 of FIG. 7A. The HEW-SIGA field 708 (e.g., the HEW-SIGA1 708-1or the HEW-SIGA2 708-2) of the data unit 900 includes a GI indication902. According to an embodiment, the GI indication 902 is set toindicate one of (i) normal guard interval, (ii) short guard interval or(iii) long guard interval. In an embodiment, the guard interval (GI)indication 902 comprises two bits, wherein a first combination of valuesof the bits indicates the normal guard interval, a second combination ofvalues of the bits indicates the short guard interval, and a thirdcombination of values of the bits indicates the long guard. Asillustrated in FIG. 9A, the normal guard interval is used for all OFDMsymbols of the preamble of the data unit 700, and one of the normalguard interval, the short guard interval, and the long guard interval,as indicated by the GI indication 902, is used for OFDM symbols of thedata portion 716, in the illustrated embodiment.

FIG. 9B is a diagram illustrating an example data unit 950 in which thenormal guard interval is used for a portion of a preamble of the dataunit, according to an embodiment. The data unit 950 is generally thesame as the data unit 900 of FIG. 9A, except that in the data unit 750includes a preamble 751 in which the guard interval indicated by the GIindication 902 is applied to OFDM symbols of a portion of the preamble751 as well as to the OFDM symbols of the data portion 716. Inparticular, in the illustrated embodiment, the normal guard interval isused for a first portion 751-1 of the preamble 701, and one of thenormal guard interval, the short guard interval, and the long guardinterval, as indicated by the GI indication 902, is used for OFDMsymbols of a second portion 751-2 of the preamble 751, in addition toOFDM symbols of the data portion 716. Accordingly, the guard intervalindicated by the GI indication 902 skips the OFDM symbol thatcorresponds to the HEW-STF 710 and is applied beginning with the OFDMsymbol that corresponds to the HEW-STF 712-1, in the illustratedembodiment. Skipping the HEW-STF 710 allows the device receiving thedata unit 950 sufficient time to decode the GI indication 902 and toproperly set up the receiver to begin decoding OFDM symbols using theguard interval indicated by the GI indication 902 prior to receivingsuch OFMD symbols, in at least some embodiments.

FIG. 10A is a diagram illustrating an example data unit 1000 in whichOFDM tone spacing is used to effectively increase guard intervalduration, according to an embodiment. The data unit 1000 is generallythe same as the data unit 900 of FIG. 7A, except that in the data unit1000, when the GI indication 902 indicates that the long GI is beingutilized, the OFDM symbols of the data portion 716 are generated usingOFDM modulation with smaller tone spacing compared to tone spacing usedfor normal guard interval OFDM symbols of the data unit 1000.

FIG. 10B is a diagram illustrating an example data unit 1050 in whichOFDM tone spacing is used to effectively increase guard intervalduration, according to another embodiment. The data unit 1050 isgenerally the same as the data unit 950 of FIG. 9B, except that in thedata unit 1050, when the GI indication 902 indicates that the long GI isbeing utilized, the OFDM symbols of the second portion 751-2 and OFDMsymbols of the data portion 716 are generated using OFDM modulation withsmaller tone spacing compared to tone spacing used for normal guardinterval OFDM symbols of the data unit 1050.

In some embodiments, a different preamble format is used for extensionguard interval mode data units compared to the preamble used for regularguard interval mode data units. In such embodiments, a device receivinga data unit can automatically detect whether the data unit is a regularguard interval mode data unit or an extension guard interval mode dataunit based on the format of the preamble of the data unit. FIG. 11A is adiagram illustrating a regular guard interval mode data unit 1100,according to an embodiment. The regular guard interval mode data unit1100 includes a regular guard interval mode preamble 1101. The regularguard interval mode preamble 1101 is generally the same as the preamble701 of the data unit 700 of FIG. 7A. In an embodiment, the preamble 1101includes a HEW-SIGA field 1108, which includes a first HEW-SIGA1 field1108-1 and a second first HEW-SIGA2 field 1108-1. In an embodiment, theHEW-SIGA field 1108 (e.g., the HEW-SIGA1 1108-1 or the HEW-SIGA2 1108-2)of the preamble 1101 includes a GI indication 1102. The GI indication1102 is set to indicate whether the normal guard interval or the shortguard interval is used for OFDM symbols of the data portion 716 of thedata unit 1100, in an embodiment. In an embodiment, the GI indication1102 comprises one bit, wherein a first value of the bit indicates thenormal guard interval and a second value of the bit indicates a shortGI. As will be explained in more detail below, a device receiving thedata unit 1100 is able to detect, based on the format of the preamble1101, that the preamble 1101 is a regular guard interval mode preamble,and not an extension guard interval mode preamble, in an embodiment.Upon detecting that the preamble 1101 is the regular guard interval modepreamble, the receiving device determines, based on the GI indication1101, whether the normal guard interval or the short guard interval isused for OFDM symbols of the data portion 716, and decodes the dataportion 716 accordingly, in an embodiment.

FIG. 11B is a diagram illustrating an extension guard interval mode dataunit 1150, according to an embodiment. The extension guard interval modedata unit 1150 includes an extension guard interval mode preamble 1151.The data unit 1150 is generally similar to the data unit 1100 of FIG.11A, except that the preamble 1151 of the data unit 1150 is formatteddifferently from the preamble 1101 of the data unit 1100. In anembodiment, the preamble 1151 is formatted such that a receiving devicethat operates according to the HEW communication protocol is able todetermine that the preamble 1151 is an extension guard interval modepreamble rather than a regular guard interval mode preamble. In anembodiment, the extension guard interval mode preamble 1151 includes anL-STF 702, an L-LTF 704, and an L-SIG 706, and one or more first HEWsignal fields (HEW-SIGAs) 1152. In an embodiment, the preamble 1150further includes one or more secondary L-SIG(s) 1154 that follow theL-SIG field 706. The secondary L-SIG(s) 1154 are followed by a secondL-LTF field (L-LTF2) 1156, in some embodiments. In other embodiments,the preamble 1151 omits the L-SIG(s) 1154 and/or the L-LTF2 1156. Insome embodiments, the preamble 1151 also includes an HEW-STF 1158, oneor more HEW-LTF fields 1160, and a second HEW signal field (HEW-SIGB)1162. In other embodiments, the preamble 1151 omits the HEW-STF 1156,the HEW-LTF(s) 1158 and/or the HEW-SIGB 1162. In an embodiment, the dataunit 1150 also includes a data portion 716 (not shown in FIG. 11B).

In one embodiment in which the preamble 1151 includes one or moresecondary L-LSIG(s) 1154, the content of each of the L-LSIG(s) 1154 isthe same as the content of the L-LSIG 706 of the data unit 1150. In anembodiment, a receiving device receiving the data unit 1150 determinesthat the preamble 1151 corresponds to an extension guard interval modepreamble by detecting the repetition(s) of the L-SIG fields 706, 1154.Further, in an embodiment, both the rate subfield and the lengthsubfield of the L-SIG 706, and, accordingly, the rate subfield(s) andthe length subfield(s) of the secondary L-SIG(s) 1154 are set to fixed(e.g., predetermined) values. In this case, upon detecting therepetition(s) of the L-SIG fields 706, 1154, the receiving device usesthe fixed values in the repeating L-SIG fields as additional traininginformation to improve channel estimation, in an embodiment. In someembodiments, however, at least the length subfield of the L-SIG 706, andaccordingly at least the length fields of the secondary L-SIG(s) 1154,is not set to a fixed value. For example, the length field is insteadset to a value determined based on the actual length of the data unit1150, in an embodiment. In one such embodiment, the receiving devicefirst decodes the L-SIG 706, and then detects the repetition(s) of theL-SIG fields 706, 1154 using the value of the length subfield in L-SIG706. In another embodiment, the receiving device first detects therepetition(s) of the L-SIG fields 706, 1154, and then combines thedetected multiple L-SIG fields 706, 1154 to improve decoding reliabilityof the L-SIG fields 706, 1154 and/or uses the redundant information inthe multiple L-SIG fields 706, 1154 to improve channel estimation.

In an embodiment in which the preamble 1151 includes L-LTF 1156, theOFDM symbol(s) of the L-LTF 1156 are generated using the long guardinterval (e.g., increased duration guard interval or decreased OFDM tonespacing guard interval). In another embodiment in which the preamble1151 includes L-LTF 11156, the OFDM symbol(s) of the L-LTF2 1506 aregenerated using the normal guard interval. For example, if a doubleguard interval (DGI) used in the L-LTF 704 is sufficiently long for thecommunication channel in which the data unit 1150 travels from thetransmitting device to the receiving device, then OFDM symbols of theL-LTF2 1506 are generated using the normal guard interval or,alternatively, the preamble 1151 omits the L-LTF 1556, in an embodiment.

In another embodiment, the preamble 1151 omits the secondary L-SIG(s)1154, but includes the L-LTF2 1156. In this embodiment, a receivingdevice detects that the preamble 1151 is the extension range preamble bydetecting the presence of the L-LTF2 1156. FIGS. 12A-12B are diagramsillustrating two possible formats of LTFs suitable for use as the L-LTF21156 according to two example embodiments. Turning first to FIG. 12A, ina first example embodiment, an L-LTF2 1200 is formatted in the samemanner as the L-LTF 704, i.e., as defined by a legacy communicationprotocol (e.g., the IEEE 802.11a/n/ac Standards). In particular, in theillustrated embodiment, the L-LTF2 1200 includes a double guard interval(DGI) 1202 followed by two repetitions of a long training sequence 1204,1206. Turning now to FIG. 12B, in another example embodiment, an L-LTF21208 is formatted differently from the L-LTF 704. In particular, in theillustrated embodiment, the L-LTF2 1208 includes a first normal guardinterval 1210, a first repetition of a long training sequence 1212, asecond normal guard interval 1214, and a second repetition of the longtraining sequence 1216.

Referring back to FIG. 11B, in an embodiment, the HEW-SIGA(s) 1152 aregenerated using the long guard interval (e.g., increased duration guardinterval or decreased OFDM tone spacing guard interval). In anembodiment, the number of the HEW-SIGAs 1152 is the same as the numberof the HEW-SGA(s) 1108 of the regular guard interval mode preamble 1101.Similarly, in an embodiment, the content of the HEW-SIGAs 1152 is thesame as the content of the HEW-SGA(s) 1108 of the regular guard intervalmode preamble 1101. In other embodiments, the number and/or the contentof the HEW-SIGAs 1152 is different from the number and/or content of theHEW-SGA(s) 1108 of the regular guard interval mode preamble 1101. Adevice receiving the data unit 1150 decodes the HEW-SIGA(s) 1152 usingthe long guard interval based on detecting that the preamble 1151corresponds to the extension guard interval mode preamble and interpretsthe HEW-SIGA(s) 1152 appropriately as defined for the extension guardinterval mode, in an embodiment.

In an embodiment in which the preamble 1151 omits the L-SIG(s) 1154and/or L-LTF2 1156, a receiving device determines whether a preamblecorresponds to the extension guard interval mode preamble 1151 or the tothe normal guard interval preamble 1101 by detecting whether theHEW-SIGA field in the preamble is generated using the long guardinterval or the normal guard interval based on auto-correlation of theHEW-SIGA field using the long guard interval and the normal guardinterval. FIGS. 13A-13B are diagrams of the HEW-SIGA 1108 of the regularguard interval mode preamble 1101 and the HEW-SIGA 1152 of the extensionguard interval mode preamble 1151, respectively, according to anembodiment. In the illustrated embodiment, the HEW-SIGA 1108 of theregular guard interval mode preamble 1101 includes a first NGI 1302, afirst HEW-SIGA field 1304, a second NGI 1306, and a second HEW-SIGAfield 1308. In the other hand, the HEW-SIGA 1152 of the extension guardinterval mode preamble 1151 includes a first LGI 1310, a first HEW-SIGAfield 1312, a second LGI 1314, and a second HEW-SIGA field 1312. In anembodiment, a receiving device performs a first auto-correlation of theHEW-SIGA field using a normal guard interval structure, such as thestructure illustrated in FIG. 13A, performs a second auto-correlationusing a long guard interval structure, such as the structure illustratedin FIG. 13B, and performs a comparison of the auto-correlation results,in an embodiment. If auto-correlation of the HEW-SIGA field using thelong guard interval produces a greater result compared to the result ofthe auto-correlation of the HEW-SIGA field using the normal guardinterval, then the receiving device determines that the preamblecorresponds to the extension guard interval mode preamble 1151, in anembodiment. On the other hand, if auto-correlation of the HEW-SIGA fieldusing the normal guard interval produces a greater result compared tothe result of auto-correlation of the HEW-SIGA field with the long guardinterval, then the receiving device determines that the preamblecorresponds to the regular guard interval mode preamble 1101, in anembodiment.

Referring again to FIG. 11B, in an embodiment, the preamble 1151 isformatted such that a legacy client station can determine a duration ofthe data unit 1150 and/or that the data unit does not conform to alegacy communication protocol. Additionally, the preamble 1151 isformatted such that a client station that operates according to the HEWprotocol is able to determine that the data unit conforms to the HEWcommunication protocol, in an embodiment. For example, at least two OFDMsymbols immediately following the L-SIG 706 of the preamble 1151, suchas the L-LSIG(s) 1154 and/or the L-LTF2 1156 and/or the HEW-SIGA(s)1152, are modulated using BPSK modulation. In this case, a legacy clientstation will treat the data unit 1150 as a legacy data unit, willdetermine a duration of the data unit based on the L-SIG 706, and willrefrain from accessing the medium for the determined duration, in anembodiment. Further, one or more other OFDM symbols of the preamble1151, such as one or more of the HEW-SIG(s) 1152 are modulated usingQ-BPSK modulation, allowing a client station operating according to theHEW communication protocol to detect that the data unit 1150 conforms tothe HEW communication protocol, in an embodiment.

In some embodiments, the HEW communication protocol allows beamformingand/or multi user MIMO (MU-MIMO) transmission in the extension guardinterval mode. In other embodiments, the HEW communication protocolallows only single stream and/or only single user transmission in theextension guard interval mode. With continued reference to FIG. 11B, inan embodiment in which the preamble 1151 includes the HEW-STF 1158 andthe HEW-LTF(s) 1160, the AP 14 applies beamforming and/or multi-usertransmission beginning with the HEW-STF 1158. In other words, the fieldsof the preamble 1151 precede the HEW-STF 1158 are omni-directional and,in multi-user mode, are intended to be received by all intendedrecipients of the data unit 1150, while the HEW-STF field 1158, as wellas the preamble fields that follow the HEW-STF field 1158 and the dataportion that follows the preamble 1151, are beam-formed and/or includedifferent portions intended to be received by different intendedrecipients of the data unit 1150, in an embodiment. In an embodiment,the HEW-SIGB field 162 includes user-specific information for theintended recipients of the data unit 1150 in MU-MIMO mode. The HEW-SIGBfield 1162 is generated using the NGI or the LGI, depending on anembodiment. Similarly, the HEW-STF 1158 is generated using the NGI orthe LGI, depending on an embodiment. In an embodiment, the trainingsequence used on the HEW-STF 1158 is the sequence defined in a legacycommunication protocol, such as in the IEEE 802.11ac protocol.

On the other hand, in an embodiment in which the preamble 1151 omits theHEW-STF 1158 and the HEW-LTF(s) 1160, beamforming and MUMIMO are notallowed in the extension guard interval mode. In this embodiment, onlysingle user single stream transmission is allowed in the extension guardinterval mode. In an embodiment, a receiving device obtains a singlestream channel estimate based on the L-LTF field 704, and demodulatesthe data portion of the data unit 1150 based on the channel estimateobtained based on the L-LTF field 704.

FIG. 14A is a block diagram illustrating an extension guard intervalmode data unit 1400, according to an embodiment. The data unit 1400includes an extension guard interval mode preamble 1401. The extensionguard interval 1401 is generally similar to the extension guard intervalmode 1151 of FIG. 11B, except that the L-SIG 706 and the secondary L-SIG1154 of the preamble 1151 are combined into a single L-SIG field 1406 inthe preamble 1401. FIG. 14B is a diagram illustrating the L-SIG field1406 according to one embodiment. In the embodiment of FIG. 14B, theL-SIG field 1406 includes a double guard interval 1410, an first L-SIGfield 1412, which includes contents of L-SIG field 706 of the preamble1151, and a second L-SIG field 1414, which includes contents of thesecondary L-SIG2 field 1154 of the preamble 1151. In variousembodiments, L-SIG field 1406 includes a length subfield set to a fixedvalue or set to a variable value, as disused above with respect to theL-SIG fields 706, 1154 of FIG. 11B. In various embodiments, redundant(repeated) bits in L-SIG field 1406 are used for improved channelestimation as discussed above with respect to L-SIG fields 706, 1154 ofFIG. 11B.

In an embodiment, a legacy client station receiving the data unit 1400assumes that the L-SIG field 1406 includes a normal guard interval. Asillustrated in FIG. 14C, the FFT window for L-SIG information bitsassumed at the legacy client station is shifted compared to the actualL-SIG field 1412, in this embodiment. In an embodiment, to ensure thatconstellation points within the FFT window correspond to BPSKmodulation, as expected by the legacy client station, and to this allowthe legacy client station to properly decode the L-SIG field 1412,modulation of the L-SIG field 1412 is phase-shifted relative to regularBPSK modulation. For example, in a 20 MHz OFDM symbol, if the normalguard interval is 0.8 μs, and the double guard interval is 1.6 μs, thenmodulation of an OFDM tone k of the L-SIG field 1412 is shifted withrespect to the corresponding OFDM tone k of the original L-SIG as can beseen from:S _(LSIG) ^((k)) =S _(SLSIG-LSIG) ^((k)) e ^(−j·2π·0.8·20/64) =S_(SLSIG-LSIG) ^((k))·(−j)  Equation 1Accordingly, in an embodiment, L-SIG field 1412 is modulated usingreverse Q-BPSK rather than regular BPSK. Thus, for example, a bit ofvalue 1 is modulated onto −j, and a bit of value 0 is modulated onto j,resulting in {j, −j} modulation rather than the regular {1, −1} BPSKmodulation, in an embodiment. In an embodiment, due to the reverseQ-BPSK modulation of the L-SIG field 1412, a legacy client station canproperly decode the L-SIG field 1412 an determine the duration of thedata unit 1400 based on the L-SIG 1412 field, in an embodiment. A clientstation that operates according to the HEW protocol, on the other hand,can auto-detect that the preamble 1401 is an extension guard intervalmode preamble by detecting the repetition of the L-SIG field 1412 or bydetecting the reverse Q-BPSK modulation of the L-SIG field within theFFT window of the legacy client station, in an embodiment.Alternatively, in other embodiments, a client station that operatesaccording to the HEW protocol detects that the preamble 1401 is anextension guard interval mode preamble using other detection methodsdiscussed above, such as based on modulation or format of the HEW-SIGAfield(s) 1152.

Referring FIGS. 11A-11B and 14A, long guard interval is used for initialOFDM symbols of both a regular guard interval mode preamble (e.g., thepreamble 1101) and a long guard interval preamble (e.g., the preamble1151 or the preamble 1401), in some embodiments. For example, referringto FIGS. 11A-11B, the L-STF field 702, the L-LTF field 704 and the L-SIGfield 706, 1154, and HEW-SIGA field 1152 is each generated using thelong guard interval, in an embodiment. Similarly, referring to FIG. 14A,the L-STF field 702, the L-LTF field 704, the L-SIG field 1406, and theHEW-SIGA(s) 1152 are generated using the long guard interval, in anembodiment. In an embodiment, a receiving device can determine whether apreamble corresponds to the regular guard interval mode preamble or theextension guard interval mode preamble based on modulation of theHEW-SIGA field 1152 (e.g., Q-BPSK) or based on an indication included inthe HEW-SIGA field 1152, in various embodiments. Further, similar to thepreamble 1151 of FIG. 11B, the preamble 1401 of FIG. 14A includes oromits the second L-LTF2 field 1156, depending on the embodiment and/orscenario.

FIG. 15 is a block diagram illustrating format of an HEW-SIGA field1500, according to an embodiment. In some embodiments, the HEW-SIGAfield(s) 1152 of the data unit 1150 or the data unit 1400 are formattedas the HEW-SIGA field 1500. In some embodiments, the HEW-SIGA field(s)1108 are formatted as the HEW-SIGA field 1500. The HEW-SIGA field 1500includes a double guard interval 1502, a first repetition of a HEW-SIGAfield 1504 and a second repetition of HEW-SIGA 1506. In an exampleembodiment, the DGI is 1.8 μs and each repetition of HEW-SIGA is 3.2 μs.In an embodiment, the repeated bits in the HEW-SIGA field 1500 are usedto increase reliability of decoding of the HEW-SIGA field 1500. In anembodiment, the format of the HEW-SIGA field 1500 is used to auto-detectan extension guard interval mode preamble based on a comparison betweenautocorrelation of the HEW-SIGA field of the preamble using the formatof the HEW-SIGA field 1500 and auto-correlation of the HEW-SIGA field ofthe preamble using the regular HEW-SIGA field format used in the regularguard interval mode, such as the format illustrated in FIG. 13A.

FIG. 16 is a flow diagram of an example method 1600 for generating adata unit, according to an embodiment. With reference to FIG. 1, themethod 1600 is implemented by the network interface 16, in anembodiment. For example, in one such embodiment, the PHY processing unit20 is configured to implement the method 1600. According to anotherembodiment, the MAC processing 18 is also configured to implement atleast a part of the method 1600. With continued reference to FIG. 1, inyet another embodiment, the method 1600 is implemented by the networkinterface 27 (e.g., the PHY processing unit 29 and/or the MAC processingunit 28). In other embodiments, the method 1600 is implemented by othersuitable network interfaces.

At block 1602, a data portion of the data unit is generated. Generatingthe data portion at block 1602 includes generating OFDM symbols of thedata portion using one of (i) a normal guard interval, (ii) a shortguard interval, or (iii) a long guard interval.

At block 1604, a preamble of the data unit is generated. The preamblegenerate at block 1604 is generated to indicate whether at least thedata portion of the data unit generated able block 1602 is generatedusing (i) the normal guard interval, (ii) the short guard interval, or(iii) the long guard interval. In various embodiments and/or scenarios,one of the preambles 701 (FIGS. 9A, 10A), 751 (FIGS. 9B, 10B), 1101(FIG. 11A), 1151 (FIG. 11B), or 1401 (FIG. 14A) is generated at block1604. In other embodiments, other suitable preambles are generated atblock 1604. In an embodiment, the preamble generated at block 1604includes an GI indication to set to indicate whether at least the dataportion is generated using (i) the normal guard interval, (ii) the shortguard interval, or (iii) the long guard interval. In an embodiment, theGI indication comprises two bits. In an embodiment, a portion of thepreamble, in addition to the data portion, is generated using the guardinterval indicated by the GI indication. In another embodiment, thepreamble generated at block 1604 is formatted such that a receivingdevice can automatically detect (e.g., without decoding) whether thepreamble corresponds to a regular guard interval preamble or to anextension guard interval mode preamble. In an embodiment, detection ofthe extension guard interval preamble signals to the receiving devicethat at least the data portion is generated using the long guardinterval.

At block 1606, the data unit is generated to include the preamblegenerated at block 1604 and the data portion generated at block 1602.

FIG. 17 is a flow diagram of an example method 1700 for generating adata unit, according to an embodiment. With reference to FIG. 1, themethod 1700 is implemented by the network interface 16, in anembodiment. For example, in one such embodiment, the PHY processing unit20 is configured to implement the method 1700. According to anotherembodiment, the MAC processing 18 is also configured to implement atleast a part of the method 1700. With continued reference to FIG. 1, inyet another embodiment, the method 1700 is implemented by the networkinterface 27 (e.g., the PHY processing unit 29 and/or the MAC processingunit 28). In other embodiments, the method 1700 is implemented by othersuitable network interfaces.

FIG. 17 is generally similar to FIG. 16, and includes like-numberedelements with FIG. 16, except that block 1604 in FIG. 16 is replaced byblock 1704 in FIG. 17. Block 1704 is similar to block 1604 except thatblock 1704 includes, when the at least OFDM symbols of the data portionare generated using the long guard interval, (i) generating two or morerepetitions of a legacy signal field, and (ii) including the two or morerepetitions of the legacy signal field in the preamble, in anembodiment. Presence of the two or more repetitions of the legacy signalfield in the preamble serves as an indication that the at least OFDMsymbols of the data portion are generated using the long guard interval,in an embodiment.

At least some of the various blocks, operations, and techniquesdescribed above may be implemented utilizing hardware, a processorexecuting firmware instructions, a processor executing softwareinstructions, or any combination thereof. When implemented utilizing aprocessor executing software or firmware instructions, the software orfirmware instructions may be stored in any computer readable memory suchas on a magnetic disk, an optical disk, or other storage medium, in aRAM or ROM or flash memory, processor, hard disk drive, optical diskdrive, tape drive, etc. Likewise, the software or firmware instructionsmay be delivered to a user or a system via any known or desired deliverymethod including, for example, on a computer readable disk or othertransportable computer storage mechanism or via communication media.Communication media typically embodies computer readable instructions,data structures, program modules or other data in a modulated datasignal such as a carrier wave or other transport mechanism. The term“modulated data signal” means a signal that has one or more of itscharacteristics set or changed in such a manner as to encode informationin the signal. By way of example, and not limitation, communicationmedia includes wired media such as a wired network or direct-wiredconnection, and wireless media such as acoustic, radio frequency,infrared and other wireless media. Thus, the software or firmwareinstructions may be delivered to a user or a system via a communicationchannel such as a telephone line, a DSL line, a cable television line, afiber optics line, a wireless communication channel, the Internet, etc.(which are viewed as being the same as or interchangeable with providingsuch software via a transportable storage medium). The software orfirmware instructions may include machine readable instructions that,when executed by the processor, cause the processor to perform variousacts.

Further aspects of the present disclosure relate to one or more of thefollowing clauses.

In an embodiment, a method for generating a data unit for transmissionvia a communication channel includes generating a data portion of thedata unit, including generating orthogonal frequency divisionmultiplexing (OFDM) symbols of the data portion using one of (i) anormal guard interval, (ii) a short guard interval and (iii) a longguard interval. The method also includes generating a preamble of thedata unit, including generating the preamble to indicate whether atleast OFDM symbols of the data portion are generated using the normalguard interval, the short guard interval, or the long guard interval.The method additionally includes generating the data unit to include thepreamble and the data portion.

In other embodiments, the method includes any suitable combination ofone or more of the following features.

Generating the preamble of the data unit includes generating a signalfield of the preamble, wherein the signal field includes a guardinterval indication set to indicate whether at least the OFDM symbols ofthe data portion are generated using the normal guard interval, theshort guard interval, or the long guard interval.

The guard interval indication comprises two bits.

Generating the preamble includes generating a first portion of thepreamble, wherein the first portion of the preamble is (i) generatedusing the normal guard interval and (ii) includes the signal field, andgenerating a second portion of the preamble using the guard intervalindicated by the guard interval indication in the signal field.

Generating the preamble of the data unit comprises generating one of (i)a regular guard interval mode preamble or (ii) an extension guardinterval mode preamble.

Generating the preamble includes formatting the preamble such that areceiving device can automatically detect whether the preamblecorresponds to the regular guard interval preamble or to the extensionguard interval preamble, wherein the extension guard interval modepreamble serves as the indication that at least the OFDM symbols of thedata portion are generated using the long guard interval when thepreamble corresponds to the extension guard interval preamble.

Generating the extension guard interval preamble includes including, inthe extension guard interval preamble, two or more repetitions of alegacy signal field, and wherein the receiving device can automaticallydetect that the preamble corresponds the extension guard interval modepreamble based on detecting the two or more repetitions of the legacysignal field.

Generating the extension guard interval mode preamble includesgenerating a non-legacy signal field to be included in the preamble, andmodulating the non-legacy signal field differently from a correspondingnon-legacy signal field in the regular guard interval mode preamble.

The receiving device can automatically detect that the preamblecorresponds to the extension guard interval mode preamble by detectingthe modulation of the non-legacy signal field.

Generating the non-legacy signal field includes generating thenon-legacy signal field using the long guard interval.

Generating the extension guard interval mode preamble includesgenerating, using the long guard interval, a non-legacy signal field tobe included in the preamble, and wherein the receiving device canautomatically detect that the preamble corresponds to the extension modeguard interval preamble by comparing results of autocorrelation of thenon-legacy signal field performed using the long guard interval andautocorrelation of the non-legacy signal field performed using thenormal guard interval.

The data unit conforms to a first communication protocol, and whereingenerating the preamble further comprises generating the preamble suchthat (i) a legacy receiver configured to operate according to a legacycommunication protocol but not according to the first communicationprotocol can determine a duration of the data unit and (ii) a receiverconfigured to operate according to the first communication protocol candetect that the data unit conforms to the first communication protocol.

In another embodiment, an apparatus comprises a network interfaceconfigured to generate a data portion of the data unit, includinggenerating orthogonal frequency division multiplexing (OFDM) symbols ofthe data portion using one of (i) a normal guard interval, (ii) a shortguard interval and (iii) a long guard interval. The network interface isalso configured to generate a preamble of the data unit, includinggenerating the preamble to indicate whether at least OFDM symbols of thedata portion are generated using the normal guard interval, the shortguard interval, or the long guard interval. The network interface isadditionally configured to generate the data unit to include thepreamble and the data portion.

In other embodiments, the apparatus includes any suitable combination ofone or more of the following features.

The network interface is further configured to generate a signal fieldto be included in the preamble, wherein the signal field includes aguard interval indication set to indicate whether at least the OFDMsymbols of the data portion are generated using the normal guardinterval, the short guard interval, or the long guard interval.

The guard interval indication comprises two bits.

The network interface is further configured to generate a first portionof the preamble, wherein the first portion of the preamble is (i)generated using the normal guard interval and (ii) includes the signalfield, and generate a second portion of the preamble using the guardinterval indicated by the guard interval indication in the signal field.

Generating the preamble of the data unit comprises generating one of (i)a regular guard interval mode preamble or (ii) an extension guardinterval mode preamble, wherein generating the preamble includesformatting the preamble such that a receiving device can automaticallydetect whether the preamble corresponds to the regular guard intervalpreamble or to the extension guard interval preamble, wherein theextension guard interval mode preamble serves as the indication that atleast the OFDM symbols of the data portion are generated using the longguard interval when the preamble corresponds to the extension guardinterval preamble.

Generating the extension guard interval preamble includes including, inthe extension guard interval preamble, two or more repetitions of alegacy signal field, and wherein the receiving device can automaticallydetect that the preamble corresponds the extension guard interval modepreamble based on detecting the two or more repetitions of the legacysignal field.

Generating the extension guard interval mode preamble includesgenerating a non-legacy signal field to be included in the preamble, andmodulating the non-legacy signal field differently from a correspondingnon-legacy signal field in the regular guard interval mode preamble.

The receiving device can automatically detect that the preamblecorresponds to the extension guard interval mode preamble by detectingthe modulation of the non-legacy signal field.

Generating the non-legacy signal field includes generating thenon-legacy signal field using the long guard interval.

Generating the extension guard interval mode preamble includesgenerating, using the long guard interval, a non-legacy signal field tobe included in the preamble, and wherein the receiving device canautomatically detect that the preamble corresponds to the extension modeguard interval preamble by comparing results of autocorrelation of thenon-legacy signal field performed using the long guard interval andautocorrelation of the non-legacy signal field performed using thenormal guard interval.

The data unit conforms to a first communication protocol, and whereingenerating the preamble further comprises generating the preamble suchthat (i) a legacy receiver configured to operate according to a legacycommunication protocol but not according to the first communicationprotocol can determine a duration of the data unit and (ii) a receiverconfigured to operate according to the first communication protocol candetect that the data unit conforms to the first communication protocol.

When implemented in hardware, the hardware may comprise one or more ofdiscrete components, an integrated circuit, an application-specificintegrated circuit (ASIC), etc.

While the present invention has been described with reference tospecific examples, which are intended to be illustrative only and not tobe limiting of the invention, changes, additions and/or deletions may bemade to the disclosed embodiments without departing from the scope ofthe invention.

What is claimed is:
 1. A method for generating a data unit fortransmission via a wireless communication channel, the methodcomprising: selecting, at a wireless network interface device, a guardinterval from a set of guard intervals including a first guard interval,a second guard interval, and a third guard interval, wherein the firstguard interval has a length that is 50% of a length of the second guardinterval, and wherein the length of the second guard interval is 50% ofa length of the third guard interval; generating, at the wirelessnetwork device, the data unit, wherein the data unit is generated toconform to a first communication protocol, wherein the data unitincludes a preamble and a data portion; generating, at the wirelessnetwork interface device, the preamble of the data unit to include: alegacy signal field, a repetition of the legacy signal field, and anon-legacy field that includes a field that indicates the selected guardinterval, wherein generating the preamble comprises generating thepreamble such that (i) a legacy receiver configured to operate accordingto a legacy communication protocol but not according to the firstcommunication protocol can determine a duration of the data unit bydecoding the legacy signal field, and (ii) a receiver configured tooperate according to the first communication protocol can detect thatthe data unit conforms to the first communication protocol; andgenerating, at the wireless network interface device, the data portionof the data unit, including generating orthogonal frequency divisionmultiplexing (OFDM) symbols of the data portion using the selected guardinterval.
 2. The method of claim 1, wherein the field that indicates theselected guard interval comprises two bits.
 3. The method of claim 1,wherein generating the preamble includes: generating a first portion ofthe preamble, wherein the first portion of the preamble is (i) generatedusing one of the first guard interval, the second guard interval, andthe third guard interval that is different than the selected guardinterval, and (ii) includes the legacy signal field, and generating asecond portion of the preamble using the selected guard interval.
 4. Themethod of claim 3, wherein the second portion of the preamble includesthe non-legacy signal field.
 5. The method of claim 1, furthercomprising: transmitting, by the wireless network interface device, thedata unit via one or more antennas.
 6. An apparatus comprising: awireless network interface device having one or more integrated circuitsconfigured to select a guard interval from a set of guard intervalsincluding a first guard interval, a second guard interval, and a thirdguard interval, wherein the first guard interval has a length that is50% of a length of the second guard interval, and wherein the length ofthe second guard interval is 50% of a length of the third guardinterval, and generate a data unit that conforms to a firstcommunication protocol, wherein the data unit includes a preamble and adata portion; wherein the one or more integrated circuits are configuredto generate the preamble of the data unit to include: a legacy signalfield, a repetition of the legacy signal field, and a non-legacy fieldthat includes a field that indicates the selected guard interval,wherein the preamble is generated such that (i) a legacy receiverconfigured to operate according to a legacy communication protocol butnot according to the first communication protocol can determine aduration of the data unit by decoding the legacy signal field, and (ii)a receiver configured to operate according to the first communicationprotocol can detect that the data unit conforms to the firstcommunication protocol; and wherein the one or more integrated circuitsare further configured to generate the data portion of the data unit,including generating orthogonal frequency division multiplexing (OFDM)symbols of the data portion using the selected guard interval.
 7. Theapparatus of claim 6, wherein the field that indicates the selectedguard interval comprises two bits.
 8. The apparatus of claim 6, whereinthe one or more integrated circuits are further configured to: generatea first portion of the preamble, wherein the first portion of thepreamble is (i) generated using one of the first guard interval, thesecond guard interval, and the third guard interval that is differentthan the selected guard interval, and (ii) includes the legacy signalfield; and generate a second portion of the preamble using the selectedguard interval.
 9. The apparatus of claim 8, wherein the second portionof the preamble includes the non-legacy signal field.
 10. The apparatusof claim 7, wherein the wireless network interface device comprises: aphysical layer (PHY) processing unit implemented on the one or moreintegrated circuits; wherein the PHY processing unit is configured to:generate the preamble of the data unit, and generate the data portion ofthe data unit.
 11. The apparatus of claim 10, wherein the PHY processingunit comprises: one or more transceivers implemented on the one or moreintegrated circuits.
 12. The apparatus of claim 11, further comprising:one or more antennas coupled to the one or more transceivers.
 13. Atangible, non-transitory computer readable medium, or media, storingmachine readable instructions that, when executed by one or moreprocessors, cause the one or more processors to: select a guard intervalfrom a set of guard intervals including a first guard interval, a secondguard interval, and a third guard interval, wherein the first guardinterval has a length that is 50% of a length of the second guardinterval, and wherein the length of the second guard interval is 50% ofa length of the third guard interval; cause a wireless network interfacedevice to generate a data unit that conforms to a first communicationprotocol, wherein the data unit includes a preamble and a data portion;cause the wireless network interface device to generate the preamble ofthe data unit to include: a legacy signal field, a repetition of thelegacy signal field, and a non-legacy signal field that includes a fieldthat indicates the selected guard interval, wherein the preamble isgenerated such that (i) a legacy receiver configured to operateaccording to a legacy communication protocol but not according to thefirst communication protocol can determine a duration of the data unitby decoding the legacy signal field, and (ii) a receiver configured tooperate according to the first communication protocol can detect thatthe data unit conforms to the first communication protocol; cause thewireless network interface device to generate the data portion of thedata unit, including generating orthogonal frequency divisionmultiplexing (OFDM) symbols of the data portion using the selected guardinterval; and cause the wireless network interface device to transmitthe data unit via a wireless communication channel.
 14. The tangible,non-transitory computer readable medium, or media, of claim 13, whereinthe field that indicates the selected guard interval comprises two bits.15. The tangible, non-transitory computer readable medium, or media, ofclaim 13, further storing machine readable instructions that, whenexecuted by the one or more processors, cause the one or more processorsto: generate a first portion of the preamble, wherein the first portionof the preamble is (i) generated using one of the first guard interval,the second guard interval, and the third guard interval that isdifferent than the selected guard interval, and (ii) includes the legacysignal field; and generate a second portion of the preamble using theselected guard interval.
 16. The tangible, non-transitory computerreadable medium, or media, of claim 15, wherein the second portion ofthe preamble includes the non-legacy signal field.